Field of the Invention
The present invention relates generally to optical materials and, more specifically, to tailored interfaces between optical materials.
Description of the Prior Art
A typical optical system will transmit, reflect, refract or otherwise modify the propagation of light or its salient properties such as intensity or polarization. The optical system can be designed to handle one or multiple distinct wavelengths or a broad spectrum of light. As light passes from one optical element to another it will experience transmission loss (known also as Fresnel loss) due to the fact that optical elements have different refractive indices from each other or from the medium by which they are surrounded. The ubiquitous situation is that of the light propagation through the air (refractive index of 1) which is going through an optical element. For a typical lens made of silica glass (low refractive index of about 1.5), the light propagating through air will experience 4% loss as it enters the lens and another 4% when it exits the lens back into the air. When considering high-index materials (chalcogenide glasses, for example, with indices in the 2.4-2.8 range) the losses are much higher, around 17% at each glass-air interface.
The transmission (Fresnel) losses at an interface are typically reduced by applying index matching fluids or an anti-reflective coating on the surface of the optical element. Typically, these coatings take advantage of the interference phenomenon that occurs in thin films and therefore they can be designed to enhance the light transmission within a defined wavelength band (wherein constructive interference takes place), therefore reducing the reflection on the interface. Another approach is to build a structure on the window surface in which the refractive index can be made to vary gradually from the air to the value of the window material. These anti-reflective surface structures (ARSS) are generally periodic in nature such as to generate strong diffraction or interference effects, and consist in a collection of identical objects such as graded cones or depressions. The distances between the objects and the dimensions of the objects themselves are smaller than the wavelength of light with which they are designed to interact. If these ARSS are periodic, they are often referred to as “motheye” surface structures, otherwise they are called “random” surface structures (random ARSS). The term “motheye” is derived from the natural world; it was observed that the eye of nocturnal insects (e.g., a moth) reflected little or no light regardless of the light wavelength or the angle at which incident light struck the eye surface. The artificially-produced structures can then reduce significantly the transmission loss from an optical interface between air and a window or a refractive optical element. They are also shown to have higher resistance to damage from high-intensity laser illumination and are easier to clean. (Lowdermilk et al., “Graded-index antireflection surface for high-power laser applications,” Appl. Phys. Lett., 36, 891 (1980).)
Surface structures can be patterned using lithography. For example, U.S. Pat. No. 4,013,465 (1977) discloses photolithography-based methods of defining periodic ARSS on glass optical substrates in a photosensitive material and transferring that ARSS into the substrate through dry etching. U.S. Pat. No. 6,855,371 (2005) describes methods for producing a periodic ARSS by applying to the substrate a coating, embossing ARSS into the applied coating with an embossing device, and curing the coating following removal of the embossing device. U.S. Pat. No. 7,214,418 (2007) contains prior art using anodized aluminum as a mask that will transfer a random ARSS on the substrate coated with the aluminum layer. Recently, microstructuring of chalcogenide fiber ends has been demonstrated, and a method of making cables with such fiber has been proposed in U.S. Patent Application 20110033156 (2010). (Sanghera et al., “Reduced Fresnel losses in chalcogenide fibers by using anti-reflective surface structures on fiber end faces,” Opt. Expr., 18, 26760 (2010).)
While ARSS have been demonstrated on a variety of substrates from sapphire and ALON to ZnSe to germanium, they are all concerned with reducing the Fresnel losses at the interface between the optical element and air, which is the medium considered typically. U.S. Pat. No. 7,903,338 (2011) discloses reducing the loss in transmission from one optical element to another by means of a stack of thin film layers.
The techniques used currently to protect sensitive optics require either the addition of foreign elements (such as epoxy) or cannot provide for a direct way to reduce the reflection loss at the optical interface.